In a delta modulator, a baseband input signal to be coded is periodically compared to the output of an integrator at a sampling frequency f.sub.s. If the input signal is larger than the integrator signal at any given sampling time, the modulator generates an output "+1". At the same time, a positive step of amplitude .sigma..sub.+ is applied to the integrator, thereby increasing its output signal for the next comparison. If, on the other hand, the input signal is smaller than the integrator signal at a particular sampling time, the modulator generates an output "-1" and a negative step of amplitude .sigma..sub.- is applied to the integrator.
An output wave, called the "idle-channel noise", is generated by a delta modulator even when its input signal is quiescent. The frequency spectrum of the idle-channel noise comprises an infinite series of components, the frequency domain locations of which are dependent on f.sub.s and a quantity referred to as the relative step size imbalance, .theta.. The total inband idle-channel noise power is minimized when the idle-channel noise fundamental frequency, .theta.f.sub.5, is located just outside the input signal baseband, Heretofore, however, it has been found difficult to control the value of .theta..
A further problem arises in pulse code modulators wherein the pulse-code modulated (PCM) signal is generated by integrating the delta-modulated signal using a digital integrator. In such PCM modulators, amplitude overload in the PCM signal, which would otherwise result from the fact that .theta. cannot as a practical matter be reduced to zero, is overcome by introducing a predetermined (and precisely controllable) leak, .beta., into the delta-modulation-to-PCM converter. However, the resultant PCM signal includes a substantial dc component which, disadvantageously, reduces the effective PCM quantization range.